The invention relates to an automatic machine tool for the production of basic structures for dental prostheses, in particular for dental crowns and/or bridges, of precise three-dimensional shape, which basic structures can be attached to prepared natural and/or artificial stumps, where the machine tool has a machine frame or housing, a workpiece carrier with a rotation shaft, at least one digitisation unit, at least one machining unit and an electronic calculating and control unit for all drive elements. The invention also concerns a process for production of positive basic structures for dental prostheses with the automatic machine tool.
A number of devices and processes are known for the production of artificial dental bridges and crowns which are collectively known as dental prostheses. In principle after dental preparation an impression of the dental stump, dental environment and jaw is taken. A system with a mouth camera is also known which derives the machining data from pictures taken in the mouth without producing impressions.
From the impression by way of a gypsum moulding, a master model can be produced. This master model shows in gypsum the situation in the patient's mouth. On this master model, the dental technician by manual skill produces a model of the basic structure of the dental prosthesis in wax and plastic which melts at low temperature or hardens by polymerisation. This model can be embedded in refractory material, baked and then cast out of metal material. The wax model—this term being also used for plastic—can also be transferred to another material by way of mechanical copy machining in scale 1:1, enlarged or reduced. Here we are interested only in the “copy machining” also with enlargement or reduction. A basic structure of ceramic for dental crowns and/or bridges is dense-sintered and shrunk into the definitive shape such that the basic structure can later be set precisely on the dental stump. By the application of a coating of porcelain (hard ceramic) or plastic on the outer surface of the basic structure, the dental crown or bridge can be individualised as required.
WO, A1 96/05782 describes a manually operable device of analog function for production of dental fillings and similar. The device contained two spindles to rotate a model and a blank. The model and blank must rotate in synchrony. Perpendicular to the axis of rotation of the model and blank, on the model is fitted a probe and on the blank a machining tool. As the model and blank rotate, the probe is brought manually into contact with the model surface. At the same time the machining tool machines the blank correspondingly. As the probe is moved over the entire model surface, a scale copy of the model is produced. The main disadvantages of this embodiment are the non-adjustable scaling i.e. the absence of an enlargement or reduction facility, the manual operation, the necessary precise matching of probe and tool, and the problems in production of bodies with cavities (concave surface form). The device described in WO, A1 96/05782 is therefore not suitable for automatic production of dental crowns and dental bridges of any geometry.
U.S. Pat. No. 5,184,306 discloses an automatic high precision production of objects with complex and individual geometry. These complex objects can for example also be dental crowns or dental bridges, over the digital data values of which are laid ideal geometries taken from a library, for example. These ideal geometries are then adapted to the digital data and changed. The paths for the machining tool are then derived from this. No device is shown as such.
EP, A2 0904742 and other publications disclose devices which consist of two separate machines, each of which has an integrated calculator system. The one device is used for digitisation of the surface of a master model, the other for machining the dental crown and/or bridge from a blank. In such devices there are numerous interfaces. The investment costs for such devices are usually high.
The digitisation of the surface of a master model gives approximately the cavital surface of the basic structure. Cementing gaps and occlusal surfaces of the basic structure must be added during calculation e.g. by way of area- or volume-derived complex three-dimensional models. The working method and working means thus do not correspond to the traditional skilful method of work of a dental technician but still require specially trained experts.
JP, A 1058281 describes a computer-controlled machine tool with a common drive for digitisation and machining units which can be exchanged or used in succession. Measuring and machining take place by means of a CAD/CAM system (computer-aided design, computer-aided manufacturing). The workpiece, preferably a blank of dental material or the model, is held by way of a rotation shaft on its facing side in a casing which is movable in the x and y direction. The measurement or machining tool is mounted to be movable in the z direction mounted on an arm of the machine tool. The measurement and machining covers the entire surface of the model or machined blank including the occlusal outer surface and cavital inner surface.
Other devices also work with CAD/CAM systems. Starting from the digitised data they must perform an area derivation or derivation of the digitised surfaces in the CAD system. Further processing of the data e.g. insertion of standardised intermediate elements from a library by way of CAD is then possible. Working with such systems requires special knowledge and skills and due to the use of standardised intermediate elements is restricted with regard to individuality for the patient situation.
The present invention is based on the task of creating an automatic machine tool of the type described initially and a process for the production of positive basic structures which allows reliable production with a small, easy to operate device. The device and process are in particular suitable for a basic structure of a porous ceramic green product—high strength ceramic after sintering—but also for basic structures of plastic or another material which is easy to machine.